Intraluminal, intracavity, intravascular, and intracardiac treatments and diagnosis of medical conditions utilizing minimally invasive procedures are effective tools in many areas of medical practice. These procedures are typically performed using imaging and treatment catheters that are inserted percutaneously into the body and into an accessible vessel of the vascular system at a site remote from the vessel or organ to be diagnosed and/or treated, such as the femoral artery. The catheter is then advanced through the vessels of the vascular system to the region of the body to be treated. The catheter may be equipped with an imaging device, typically an ultrasound imaging device, which is used to locate and diagnose a diseased portion of the body, such as a stenosed region of an artery. For example, U.S. Pat. No. 5,368,035, issued to Hamm et al., the disclosure of which is incorporated herein by reference, describes a catheter having an intravascular ultrasound imaging transducer.
FIG. 1 shows an example of an imaging transducer assembly 1 known in the art. The imaging transducer 1 is typically within the lumen 60 of a guidewire (partially shown), having an outer tubular wall member 5. The imaging transducer assembly 1 includes a coaxial cable 110, having a center conductor wire and an outer shield wire (not shown). A conductive wire, having a diameter of approximately 500 microns, is wrapped around the coaxial cable 110, forming a coil, which functions as a drive shaft 10. Connected to the distal end of the drive shaft 10 is a stainless steel housing 20, which serves to reinforce the structure of the imaging transducer assembly 1. Surrounding the coaxial cable 110, within the housing 20 is a silver epoxy 30, a conductive material. Thus, the housing 20 is electrically coupled to the shield wire of the coaxial cable 110 via the epoxy 30. On the distal end of the silver epoxy 140 is an insulating substance, a non-conductive epoxy 35.
On the distal end of the non-conductive epoxy 35 is a layer of piezoelectric crystal (“PZT”) 80, “sandwiched” between a conductive acoustic lens 70 and a conductive backing material 90, formed from an acoustically absorbent material (e.g., an epoxy substrate having tungsten particles). The acoustic lens 70 is electrically coupled with the center conductor wire of the coaxial cable 110 via a connector 40 that is insulated from the silver epoxy 30 and the backing material 90 by the non-conductive epoxy 35. The backing material 90 is connected to the steel housing 20. It is desirable for the imaging transducer assembly 1 to be surrounded by a sonolucent media. Thus, the lumen 60 of the guidewire is also filled with saline around the assembly 1. The driveshaft 10, the housing 20, and the acoustic lens 70 are exposed to the saline. During operation, the PZT layer 80 is electrically excited by both the backing material 90 and the acoustic lens 70. The backing material 90 receives its charge from the shield wire 140 of the coaxial cable 110 via the silver epoxy 30 and the steel housing 30, and the acoustic lens 70, which may also be silver epoxy, receives its charge from the center conductor wire 120 of the coaxial cable 110 via the connector 40, which may be silver epoxy as well.
In some instances, it may be desirable to be able to obtain not only the cross-sectional image of a blood vessel, but also information such as the three-dimensional longitudinal profile of the same blood vessel. One approach in obtaining such additional information is to use a medical positioning system, which is generally known in the art. Turning to FIG. 2a, a prior art medical positioning system 240 is illustrated. The system 240 generally includes a plurality of transmitter and/or receiver nodes 250 that may be arranged around a patient. For instance, the nodes 250 may be arranged on a framework of towers that surround a patient. The system 240 further includes one or more sensors 260, which are configured to send and/or receive electromagnetic, or electromechanical, signals to and/or from the transmitter/receiver nodes 250.
A sensor 260, coupled with a guidewire (partially shown), may be placed within the blood vessel of a patient's body. The signals exchanged between the sensor 260 and the nodes 250 function as navigational signals which, as can be appreciated by one of ordinary skill in the art, may be used to determine the position of the sensor 260 within the patient's body. In other words, the sensor 260 transmits navigational signals to the nodes 250, and a processor (not shown) coupled with the nodes 250 determines the position of the sensor 260 based on the signals received by the nodes 250. Alternatively, or in addition, the nodes 250 may send navigational signals to the sensor 260, and a processor (not shown) coupled with the sensor 260 determines the position of the sensor 260 within the patient's body based on the signals sent by the nodes 250. The medical positioning system 240 can track and record the position of the sensor 260 as it is moved throughout a patient's blood vessel, thus providing a longitudinal profile of the blood vessel.
As is known in the art, a sensor of a medical positioning system may be combined with an imaging transducer to form a transducer/sensor assembly 300. Turning to FIG. 2b, a cross-sectional side view of an example transducer/sensor assembly 300 is shown in a lumen 305 of the distal portion of a guidewire or catheter assembly (partially shown) having an outer tubular wall 301. The transducer/sensor assembly 300 includes an imaging transducer 340, such as that described above, and a sensor 320 of a medical positioning system. The “antenna” portion of the sensor 320 is an insulated conductive wire 325. The wire 325 may also have magnetic qualities. The wire 325 is tightly wrapped around a portion of the distal end of the coaxial cable 410 and non-conductive epoxy 330, and is also tightly wrapped around the distal end of the drive shaft 310, forming a second coil shape. The second coil shape desirably provides an inductance for the antenna portion of the sensor 320 when charged to increase its ability to send and receive electromagnetic signals. A more detailed description of a catheter having a combined transducer/sensor assembly is provided in U.S. patent application Ser. No. 10/401,901, filed on Mar. 28, 2003, which is hereby incorporated by reference in its entirety.
The environment within which the imaging catheter operates typically includes other electronic devices, such as an electrocardiogram (“EKG”) system or other monitors, which are situated fairly close to the catheter so the technician has convenient access to all the devices. However, each of these devices generate an electromagnetic field, and if they are situated sufficiently close, the respective fields can cause signal distortion in other devices. Accordingly, an improved imaging system is desirable.